1. Field of the Invention
The present invention relates generally to frequency mixers, and more particularly to subharmonic mixers employing low conversion loss techniques for millimeter wavelength systems.
2. Discussion of Related Art
To optimally detect radio frequency (RF) signals, most electronic receivers convert received RF signals to lower intermediate frequencies. The filtering and amplification performed in detecting intermediate frequencies require less expensive electrical components than those required for accurate detection at RF. Detection at intermediate frequencies also can improve receiver gain, dynamic range, and stability.
Typical RF receivers employ mixers to convert the received RF signal to a lower intermediate frequency. Frequency shifting occurs by mixing, or taking the difference between, the received RF signal and a reference frequency from a local oscillator. The difference between a desired frequency within the received RF signal and the local oscillator frequency is the intermediate frequency. That is, the desired frequency is setoff from the local oscillator frequency by the intermediate frequency.
The received RF signal, however, may also contain noise and other interfering signals. If the noise or the other interfering signals are setoff from the local oscillator frequency by the intermediate frequency, the mixing of the noise or the other interfering signals with the local oscillator frequency will also produce noise at the intermediate frequency. This noise or other interfering signals may be at the image frequency and can degrade the overall noise figure of the mixer by about 3 dB for single sideband operation.
Mixers also generate harmonics and other mixing products of the local oscillator signal. Some of these products, e.g., the sum frequency (signal plus the local oscillator) and the image (twice the local oscillator minus the signal) are derived from the desired frequency. If these products are not remixed with the local oscillator frequency and converted to the intermediate frequency, the mixer will exhibit an increased conversion loss.
Conventional approaches to alleviating an increase in noise figure due to amplified image frequency noise have involved either filtering or phasing techniques. For receiver systems having narrow RF bandwidths and high intermediate frequencies, a filter is often inserted between the front end amplifier and the mixer to attenuate the image frequency noise. For broad RF bandwidths and/or low intermediate frequencies, image rejection by phasing to separate and terminate the intermediate frequency caused by the image frequency has been used.
To lower the conversion loss caused by self-generated products such as the image frequency, conventional mixers have employed image enhancement techniques. These techniques have attempted to reflect the image back into the mixer for conversion to the intermediate frequency.
Conventional image rejection and image enhancement techniques, however, do not encompass the necessities of emerging millimeter wavelength systems. In such systems, a local oscillator frequency that is set off from the desired frequency by the intermediate frequency might have to be upwards of 94 GHz. This local oscillator frequency may not be readily available. If a sub-multiple of that local oscillator frequency is used, a frequency multiplier is required to adjust the sub-multiple up to the local oscillator frequency.
In light of the foregoing, there is a need for a harmonic frequency mixer for millimeter wavelength systems that does not require a separate multiplier and that incorporates image rejection and image enhancement to achieve low conversion loss while being pumped by an oscillator at a sub-multiple of the usual local oscillator frequency.